Breastfeeding and West Nile Virus

Breastfeeding mothers have been concerned about West Nile Virus (WNV) since the flurry of publicity in Fall 2002 about the transmission of West Nile via breastmilk.

The current recommendation from the US Centers for Disease Control is that a mother should not interrupt breastfeeding if she contracts West Nile Virus, since the risk to the baby from weaning to infant formula is greater than the risk posed by the potential WNV infection.

Components of breastmilk (IgG, IgM, lipids) have been shown in laboratory tests (in vitro) to be active against West Nile Virus. See Antiviral factors found in human milk for more information.

Live West Nile Virus (the intact, infectious form of the virus) has not been cultured from breastmilk. Fragments of genetic material (RNA) and WNV-specific antibodies have been cultured from breastmilk.

In September 2002, a woman contracted West Nile Virus from a postpartum blood transfusion. Her newborn was exclusively breastfed and exposure to WNV through anything other than breastmilk was unlikely.

At 16 days postpartum, the mother’s breastmilk tested positive for WNV antibodies (IgM, IgG) and WNV genetic material (RNA, not the live virus).

At 17 days postpartum, the mother discontinued breastfeeding.

At 24 days postpartum, the mother’s milk tested negative for WNV genetic material (RNA), and positive for WNV antibodies (IgM).

Per the CDC:
“Although WNV genetic material was present transiently in breast milk, the specific timing and source of the infant’s infection remain unclear… Until live virus is cultured from breast milk, or until definitive data are obtained to document WNV transmission through breast milk, the findings described in this report should be interpreted with caution.”
– source: CDC. Possible West Nile Virus Transmission to an Infant Through Breast-Feeding — Michigan, 2002. MMWR 2002;51(39);877-878.

Per the CDC:
“WNV RNA has not been identified previously in breast milk, and no studies are known that define the implications of this laboratory finding. Laboratory investigations, including attempts to culture WNV from additional breast milk samples, are under way. Until live virus is cultured from breast milk, or until definitive data are obtained to document WNV transmission through breast milk, the TaqMan® findings described in this report should be interpreted with caution… The risk for transmission of WNV from mother to infant through breast-feeding is unknown. The infant described in this report remains healthy despite breast-feeding for 17 days. Until follow-up testing on the infant is completed, it is unknown whether the infant was infected with WNV. The health benefits of breast-feeding are well established, and these findings do not suggest a change in breast-feeding recommendations.”
– source: CDC. Update. Investigation of West Nile Virus Infections in recipients of Organ Transplantation and Blood Transfusion—Michigan, 2002. MMWR 2002;51:879.

The baby remained healthy throughout and had no symptoms of West Nile Virus.

Keep in mind that in many cases where mom has been exposed to West Nile Virus via mosquito bites (the primary route of infection), baby has also been in contact with those same mosquitos.

If mom does become infected with WNV, she will likely not know about it until symptoms appear (3-14 days), and baby will have already been exposed to the virus at that point (via a mosquito bite and/or breastmilk). Breastfeeding builds baby’s immune system and will also provide baby with extra antibodies to West Nile Virus.

Discontinuing breastfeeding would deprive baby of the extra immune protection from breastfeeding, and is not advised.

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